The austenitic chromium-nickel and chromium-nickel- molybdenum stainless steels are used in a variety of corrosion-resistant parts and fittings. The manufacture of many of these parts and fittings requires considerable machining, and thus the machinability as well as the corrosion resistance of these austenitic stainless steels is an important factor affecting their use in these applications.
It is well known that the machinability of the chromium-nickel and chromium-nickel-molybdenum stainless steels can be improved by the addition of sulfur, selenium, tellurium, bismuth, lead, and phosphorus. However, the addition of sulfur and of these other elements adversely affects corrosion resistance and the ability of these stainless steels to be continuously cast or hot worked without undue difficulty.
Efforts have been made to improve the machinability of the austenitic stainless steels without sacrificing corrosion resistance by adding small amounts of sulfur to achieve the greatest possible improvement in machinability without unduly reducing corrosion resistance. In this regard, U.S. Pat. No. 3,563,729 discloses that austenitic stainless steels having improved machinability without a notable sacrifice in corrosion resistance can be achieved by the addition of 0.04 to 0.07 percent sulfur. While such austenitic stainless steels are very useful, many applications exist where the combination of machinability and corrosion resistance afforded by them is not satisfactory, and where still better machinability is desired without a decrease in corrosion resistance. Further, as with other sulfur-bearing austenitic stainless steels, they suffer from the disadvantage that when continuously cast their machinability is adversely affected by the tendency of this casting technique to produce more numerous and smaller sulfide inclusions than achieved by conventional ingot casting.
It has now been discovered that the machinability of the austenitic chromium-nickel and chromium-nickel-molybdenum stainless steels with either low or slightly elevated sulfur contents can be improved by maintaining carbon and nitrogen, in combination, at lower than conventional levels and by controlling silicon at an optimum level. An important advantage of this discovery is that machinability can be improved without a decrease in corrosion resistance. Further, in contrast to those austenitic stainless steels in which sulfur is the primary agent used to improve machinability, the steels of this invention can be continuously cast without difficulty and without significantly decreasing their machinability.